This invention relates to X-ray sources and, more particularly, to a monitoring of the speed of rotation of a target and the aging of a radiation surface thereof.
X-ray sources, such as those utilized in the field of medicine for the imaging of subjects, frequently employ a rotating target bombarded by a beam of electrons from a cathode. The beam of electrons is directed to a focal track on an inclined surface of the target from which X-radiation radiates in response to the impingement of the electrons upon the radiating surface. The surface from which the radiation radiates may be referred to hereinafter as the radiating surface. Rotation of the target past the electron beam distributes the heat induced by the beam along the entire focal track thereby preventing the build up of excessive heat at any one point on the target. The distribution of the heat along the focal track allows relatively higher power densities of the X-radiation as compared to the power density attainable with a stationary target.
A problem arises in the operation of such X-ray sources in that knowledge of the speed of rotation of the target is not readily available without the use of some sort of detector of the speed of rotation. The rotation data may be utilized for determining when the power is to be increased in the anode-cathode circuit. Unfortunately, devices which measure shaft angle rotation, such as the rotation of the shaft upon which the target is mounted, require the installation of an optical, mechanical or electrically responsive device along the shaft itself which, in the case of an X-ray source, would necessitate an invasion of the housing of the source in order to install such a detection device. The application of the power is preferably delayed until the rotation of the target has attained the desired rate. In addition, the rotational data may be employed to insure that the source is not operated at the rotational speed of a mechanical resonance of the target and the rotor of its drive motor. Also, the foregoing rotation measurement devices fail to provide any data as to the condition of the radiating surface of the target.